Magnetic Circular Dichroism Response Research Articles

The visible magneto-optical response of RE1-xAxMnO3manganites: relationship with the charge component of the material.

Magnetic circular dichroism (MCD) spectroscopy for manganite films of various compositions and morphologies has been studied in the range of 1.2-3.7 eV. The primary focus was on the temperature behavior of the MCD spectra, as well as the magnetization and resistivity of the films. The data obtained were analyzed in comparison with magneto-optical spectroscopy of the Kerr rotation (KR) on both single crystal and thin film of manganites. It has been established that the MCD response at 2.3 eV is typical for manganites transitioning into a conducting state. Consequently, it reflects a change in the band structure of the material. This response is also observed in the KR spectrum of manganites in the range 2.3-2.6 eV below the metal-insulator transition temperature. These findings complement the understanding of the electronic structure of manganites in general. Moreover, they also provide a basis for the search for new functional materials.

Magneto-optical properties of superparamagnetic CoPt alloy nanoparticles in the UV–visible range

Superparamagnetic CoPt alloy nanoparticles were synthesized via a wet chemical method and exhibited intense magnetic circular dichroism (MCD) in the UV–visible range. The dissymmetry factor of MCD, gMCD, for the CoPt nanoparticles was 0.034 at room temperature in a magnetic field of ±1.6 T. The MCD responses may be due to plasmonic circular currents generated in the metallic CoPt nanoparticles by circularly polarized light. The responses were higher than those of the Co nanoparticles, likely due to the chemical stability and spin–orbit coupling.

UV-resonant magnetoplasmonic properties of chemically synthesized indium nanoparticles.

In this article, we for the first time demonstrate magnetoplasmonic properties of metallic indium (In0) nanoparticles, one of the promising non-noble metals for UV plasmonics, by using magnetic circular dichroism (MCD) spectroscopy. In0 nanoparticles are synthesized by a kinetically controlled reduction of indium salts, followed by performing centrifugation-based size selection, giving nanoparticles of 37.5 ± 9.7 or 51.6 ± 8.4 nm in diameter. These In0 nanoparticles exhibit a single extinction peak in the UV region (<300 nm), which can be attributed to localised surface plasmon resonance (LSPR), and upon increasing the particle size, the peak is red-shifted and broadened. The MCD signatures are then typical for circular magnetoplasmonic modes of metal nanospheres, and on the basis of the MCD responses, the effective mass of an electron (m*) of indium is estimated. Interestingly, although the large-sized In0 nanoparticles (51.6 nm) have a broader LSPR linewidth, the magneto-optical (MO) activity is larger than that of the smaller one (37.5 nm), which is unlike the behaviour of Ag nanospheres with high-quality-factor plasmonic performance. This can probably be due not only to the formation of stiff semiconducting (In(OH)3) shell layers on the In0 cores (= In0@In(OH)3 core-shell morphology) but also to the effect of the dielectric function of In0 that can influence the light-helicity-dependent field-induced cyclotron shift.

Near-Infrared Optical Properties of Thiol-Passivated Substoichiometric WO3–x Nanodots Studied by Magnetic Circular Dichroism (MCD) Spectroscopy

We report a facile method to synthesize oxygen-deficient tungsten oxide (WO3–x) nanodots passivated by thiols. Addition of thiolated amino acid glutathione (GSH) or penicillamine (Pen) into ethanolic WCl6 solution gives blue WO3–x nanodot dispersion with the absorption maximum of ∼900 nm; however, complete isolation of the as-synthesized products based on precipitation–redispersion cycles makes them insoluble in any solvent. We then adopt water-based ultrafiltration for purification. Interestingly, after ultrafiltration, the optical absorption is significantly modified, having a broad peak at 1300–1500 nm with a shoulder at ∼950 nm. Magnetic circular dichroism (MCD) investigations reveal that (i) the near-infrared (NIR) absorption peak (at ∼900 nm) observed in the as-synthesized products arises from localized surface plasmon resonance, giving a derivative-like MCD response, and (ii) the shoulder observed at ∼950 nm in the ultrafiltrated products is due to the excitation to a polaronic state, giving a single-peaked MCD signal. The results suggest that the plasmonic state in the WO3–x nanodots is easily and significantly modulated by the synthesis/environmental conditions, and thus, care should be taken to account appropriately for the NIR spectroscopic properties.

A Long-Lived Water-Soluble Phenazine Radical Cation.

Long-lived water-soluble organic radical species have long been desired for applications in bioimaging and aqueous energy storage technologies. In the present work, we report a phenazine radical cation sodium 3,3'-(phenazine-5,10-diyl)bis(propane-1-sulfonate) (PSPR) with a high solubility of 1.4 M and high stability in water. Collaboratively demonstrated by experiments and theoretical calculations, PSPR is not prone to undergo dimerization or disproportionation reactions, and its appropriate electron density avoids reactions with oxygen or water, which contribute together to its long lifetime in water under air. With an open-shell configuration, PSPR shows interesting magnetic activity with a narrow linewidth in the electron paramagnetic resonance spectra and a magnetic circular dichroism response. PSPR exhibits an ambipolar redox activity in water. By pairing with a cheap zinc negative electrolyte, a high-performance aqueous organic redox flow battery based on PSPR as a positive electrolyte with an open-circuit voltage of 1.0 V is established, which shows no obvious capacity fade after cycling for 2500 cycles (∼27 days), demonstrating the great promise of PSPR for large-scale energy-storage technology.

Probing Through-Bond and Through-Space Interactions in Singlet Fission-Based Pentacene Dimers.

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the "bridge" affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems.

Unveiling the presence of metallic Co in chemically fabricated Au@CoO core-shell nanoparticles by magnetic circular dichroism (MCD) spectroscopy

In this article, we focus on optical and magneto-optical properties of Au@CoO core-shell nanoparticles synthesized by a solution-phase seeded growth route under ambient atmosphere, in which the pyrolysis of Co(acac)2 in oleylamine is conducted in the presence of preformed Au nanoparticles. Optical absorption exhibits a broad peak that can be ascribed to localized surface plasmon resonance (LSPR) of the Au counterpart, but its position is red-shifted as compared to that of pure Au nanoparticles, meaning that the Au cores are coated by a dielectric material (=CoO) with high permittivity. Nanoscopic analysis based on X-ray photoelectron spectroscopy (XPS) only weakly implies the slight existence of zero-valent Co metal. In contrast, magnetic circular dichroism (MCD) shows a very intense characteristic response that is completely different from those of both Au and CoO counterparts but close to metallic Co, demonstrating that the MCD response is dominated by a very small amount of metallic Co within the core-shell nanostructures. Hence MCD analysis can be a very efficient way with a great potential for definitely identifying the presence of slight ferromagnetic components in non-magnetic nanostructures.

Chiral Magneto-Optical Properties of Supra-Assembled Fe3O4 Nanoparticles.

Research on the chiral magneto-optical properties of inorganic nanomaterials has enabled novel applications in advanced optical and electronic devices. However, the corresponding chiral magneto-optical responses have only been studied under strong magnetic fields of ≥1 T, which limits the wider application of these novel materials. In this paper, we report on the enhanced chiral magneto-optical activity of supra-assembled Fe3O4 magnetite nanoparticles in the visible range at weak magnetic fields of 1.5 mT. The spherical supra-assembled particles with a diameter of ∼90 nm prepared by solvothermal synthesis had single-crystal-like structures, which resulted from the oriented attachment of nanograins. They exhibited superparamagnetic behavior even with a relatively large supraparticle diameter that exceeded the size limit for superparamagnetism. This can be attributed to the small size of nanograins with a diameter of ∼12 nm that constitute the suprastructured particles. Magnetic circular dichroism (MCD) measurements at magnetic fields of 1.5 mT showed distinct chiral magneto-optical activity from charge transfer transitions of magnetite in the visible range. For the supraparticles with lower crystallinity, the MCD peaks in the 250-550 nm range assigned as the ligand-to-metal charge transfer (LMCT) and the inter-sublattice charge transfer (ISCT) show increased intensities in comparison to those with higher crystallinity samples. On the contrary, the higher crystallinity sample shows higher MCD intensities near 600-700 nm for the intervalence charge transfer (IVCT) transition. The differences in MCD responses can be attributed to the crystallinity determined by the reaction time, lattice distortion near grain boundaries of the constituent nanocrystals, and dipolar interactions in the supra-assembled structures.

MCD and MCPL Characterization of Luminescent Si(IV) and P(V) Tritolylcorroles: The Role of Coordination Number.

Two triarylcorrole complexes, (hydroxy)[5,10,15-tritolylcorrolato]silicon-(TTC)Si(OH) and (dihydroxy)[5,10,15-tritolylcorrolato]phosphorous-(TTC)P(OH)2, have been investigated by magnetic circular dichroism (MCD) and magnetic circularly polarized luminescence (MCPL). The spectroscopic investigations have been combined with explicit calculation of MCD response through time-dependent density functional theory (TD-DFT) formalism. This has allowed us to better define the role of molecular orbitals in the transitions associated with the Soret and Q bands. Besides and more importantly, MCD has made it possible to follow the titration process of (TTC)Si(OH) in dimethyl sulfoxide (DMSO) solution with NaF and of (TTC)P(OH)2 in dichloromethane solution with alcohols in a complementary and, we dare say, more sensitive way with respect to absorption and fluorescence data. Finally, the MCPL spectra and the ancillary TD-DFT calculations have allowed us to characterize the excited state of (TTC)Si(OH).

Magnetic Circular Dichroism Responses with High Sensitivity and Enhanced Spectral Resolution in Multipolar Plasmonic Modes of Silver Nanoparticles with Dimensions between 90 and 200 nm.

Characteristic features of magnetoplasmonic responses in higher-order multipolar (quadrupolar and octupolar) modes of Ag nanoparticles (from 90 to 200 nm in diameter) are demonstrated for the first time using magnetic circular dichroism (MCD) spectroscopy. In optical extinction spectra, with an increase in the size of the nanoparticles, the red shift of dipolar plasmon peaks and the appearance of higher-order multipolar resonances can reasonably be observed. Aside from the dipolar and quadrupolar modes, the octupolar plasmonic extinction is very weak and almost unresolved. In contrast, strikingly, MCD shows a very sharp and intense peak (or valley) for the octupolar resonance, meaning its unique properties with high sensitivity and enhanced spectral resolution. MCD responses assignable to the quadrupolar mode have a distinct derivative-like shape, which is different from those observed for Ag nanocubes and nanodecahedra in our previous studies. We then discuss this behavior from the viewpoint of size and/or polyhedral shape inhomogeneity.

Optical and magneto-optical properties of rhodium nanostructures with different morphologies: Insight into the absorption bump in the UV region

In this study, magneto-optical properties of Rh nanostructures including nanocubes, nanomultipods, and nanotetrahedra, as promising candidates for UV plasmonics, are studied using magnetic circular dichroism (MCD) spectroscopy to obtain better insight into the characteristic absorption bump observed in the UV region. In any nanostructure, unexpectedly, the MCD responses are different from those ascribed to typical localized surface plasmon resonance (LSPR) behaviors in their sign and profile, indicating the distinct absence of LSPR signals. Simultaneous deconvolution of absorption and MCD suggests an appreciable contribution of size and/or morphology-dependent interband transitions of rhodium to their responses.

On the electronic transitions of α-Fe2O3 hematite nanoparticles with different size and morphology: Analysis by simultaneous deconvolution of UV–vis absorption and MCD spectra

Nanostructures of the naturally-existing most stable iron oxide, hematite (α-Fe2O3), gain significant research interest in various recent applications. In this study, we put the main focus on magneto-optical (MO) properties of hematite nanoparticles synthesized by a solvothermal approach, and the effects of size, morphology, and particle aggregation on the magnetic circular dichroism (MCD) and conventional absorption spectra are investigated. Simultaneous deconvolution of UV–vis absorption and MCD spectra allows us to discuss the nature of electronic transitions of various hematite nanoarchitectures on the basis of possible transitions in the 3d5 system, giving enhanced spectral resolution and detailed assignments. Consequently, we find (i) characteristic size- and shape-dependent MCD responses at 450–470 nm; and (ii) the presence of both spin-allowed pair excitation and Laporte-forbidden d–d transition with opposite MCD signs in close energy at around 550 nm.

Magnetic circular dichroism in plasmonic Ag–Au core–shell nanoparticles: how does the magneto-optical activity tune?

Magnetic circular dichroism (MCD) is demonstrated for plasmonic bimetallic nanoparticles. They have an Ag–Au core–shell configuration, which can be synthesized using a seeded growth technique. In the seeded growth process, when the amount of Au (as coating metal) added to the Ag core is small, both ordinary (lower-energy) and extraordinary (higher-energy) modes of localized surface plasmon resonance (LSPR) are detected in optical extinction spectra, suggesting successful formation of well-defined core–shell morphology with a rather thin shell (= AgAu alloy) thickness. Interestingly, the related MCD, showing a typical bisignate magnetoplasmonic response, is more distinct than the extinction signal in the ordinary LSPR mode, but strong damping is found upon thin shell formation. Hence, such bimetallic nanosystems have an advantage of systematic tuning of their magnetoplasmonic behavior, but may not be favorable to obtaining large magneto-optical (MO) activity or MCD amplitudes. The damped MCD response is discussed from a viewpoint of spectral inhomogeneity.

Dominant role of iron oxides in magnetic circular dichroism of plasmonic-magnetic Au-Fe3−xO4 heterodimer nanostructures

Plasmon-tuned magneto-optical (MO) properties enable design concepts of MO devices with various nanoarchitectures. In this study, magnetic circular dichroism (MCD) is presented in plasmonic-magnetic Au-Fe3−xO4 heterodimer nanostructures dispersed in solution. The heterodimers are synthesized by thermal decomposition of Fe(III) precursors in the presence of preformed Au nanoparticles. The extinction spectrum exhibits a localized surface plasmon resonance (LSPR) peak, which is red-shifted with respect to the isolated Au particle resonance. On the other hand, the MCD response is overwhelmingly dominated by iron oxide counterparts consisting of multiphase magnetite (Fe3O4)/maghemite (γ-Fe2O3). No appreciable LSPR-enhancement in the MO response is found. As for the effect of the magnetic component on the LSPR behavior of Au, a decrease in the magnetoplasmonic response is observed. This suggests a reduction of the effective magnetic field to the Au counterparts, which can probably be due to magnetic shielding by the iron oxides in the near vicinity of Au nanoparticles.

Magnetic Circular Dichroism of Substoichiometric Molybdenum Oxide (MoO3–x) Nanoarchitectures with Polaronic Defects

Magnetic circular dichroism (MCD) is demonstrated for the first time in substoichiometric molybdenum oxide (MoO₃–ₓ) nanoarchitectures to explore the origin of their near-IR (NIR) transitions. Various nanostructures of MoO₃–ₓ are synthesized by a simple hydrothermal process using an ionic Mo(VI) or metallic Mo(0) precursor. The phase, morphology, and spectroscopic properties of the obtained nanostructures are dependent both on the reaction temperature and the molybdenum precursor. In particular, hexagonal MoO₃–ₓ nanostructures are colored blue with different degrees. To better understand the nature of the electronic states in these nanomaterials, MCD spectroscopy is conducted. A derivative-like MCD response is detected in the vis-NIR region, but it is not attributed to surface magnetoplasmonic modes because of the absence of the peak-energy shift with the increasing refractive index of the dispersion medium. Then, the bisignate MCD signal can be attributed to Faraday B-terms for small-polaronic transitions, arising from two interacting polaronic states close in energy that would give opposite signs under an applied magnetic field. We believe that MCD evaluation for polaronic nanomaterials will expand promising new applications in fields such as semiconductor-based nanophotonics and magneto-optical devices.

Magnetic circular dichroism (MCD) in silver nanocubes with different sizes

Magnetic circular dichroism (MCD) spectroscopy is demonstrated for Ag nanocubes with different edge-lengths. A large bisignated MCD response is typical in the dipolar localized surface plasmon resonance (LSPR) region, which can be explained within a model on the magnetic Lorentz force imparted on the collective electrons that circularly move in the nanocube. In contrast, the MCD signal attributed to multipolar (including quadrupolar) LSPR exhibits a sharp simple (single) peak, but it never corresponds to the extinction peak position. This discrepancy is discussed on the basis of energetically unequal MCD responses for the left- and right-circularly polarized light excitations.

Multipolar Surface Magnetoplasmon Resonances in Triangular Silver Nanoprisms studied by MCD Spectroscopy

Magneto-optical responses of triangular silver nanoprisms are demonstrated for the first time with magnetic circular dichroism (MCD) spectroscopy. In Ag nanoprisms with the mean aspect ratio ranging from 6.3 to 8.8, optical absorption exhibits a strong main peak at 650–800 nm that is ascribed to the in-plane dipolar localized surface plasmon resonance (LSPR), along with other two peaks at 450–500 nm and ∼330 nm, which can be assigned to the in-plane and out-of-plane quadrupolar LSPR, respectively. A derivative-like MCD signal is observed at the energy of in-plane dipolar LSPR, but the position crossing zero is not in correspondence with the absorption maximum. In contrast, interestingly, the MCD response for in-plane or out-of-plane quadrupolar LSPR shows a simple (single) peak that corresponds to the position of its absorption maximum, suggesting that MCD spectroscopy is very effective for easy identification (or rapid determination) whether the LSPR modes are dipolar or quadrupolar. The agreement betwee...

Finite Size Effect on Magneto-Optical Responses of Chemically Modified Fe3O4 Nanoparticles Studied by MCD Spectroscopy

A magneto-optical study of chemically modified magnetite (Fe3O4) nanoparticles is presented with magnetic circular dichroism (MCD) spectroscopy. The magnetite nanoparticles are synthesized using the high-temperature solution reduction of Fe(III) acetylacetonate in the presence of oleylamine. The size of the magnetite nanoparticles examined is 3–4 or 7–8 nm. In contrast to broad and featureless UV–vis–NIR absorption spectra for these magnetite nanoparticles, the MCD responses are much more structured. MCD essentially corresponds to electronic transitions in the absorption spectrum, so to decode electronic transitions in the magnetite nanoparticles and to compare them with those in the bulk phase, simultaneous deconvolution analyses of both the electronic absorption and MCD spectra are conducted, giving accurate transition energies with enhanced spectral resolution. The decomposed transitions are successfully assigned on the basis of the theoretical band structure calculations previously reported. Then the relative absorption and MCD intensities of some deconvoluted bands associated with LMCT or ISCT transitions are found to be size dependent, which can be due to partial surface hydration and/or oxidation of the magnetite nanoparticles. We also compare the magneto-optical responses of magnetite nanoparticles and those of bulk Fe3O4 calculated.

On the Electronic Structures of Au25(SR)18 Clusters Studied by Magnetic Circular Dichroism Spectroscopy.

The first magnetic circular dichroism (MCD) spectra are reported at room temperature for well-defined thiolate-protected Au25 clusters (Au25(SG)18 and Au25(PET)18, where SG and PET denote glutathione and 2-phenylethanethiolate, respectively). MCD essentially corresponds to electronic transitions in the absorption spectrum, so the electronic structures of the Au25 clusters are explored based on a simultaneous deconvolution analysis of both the electronic absorption and MCD spectra, giving enhanced spectral resolution. We then find that the observed MCD responses are entirely interpreted in terms of the Faraday B terms, representing strict nondegeneracies of the excited and ground states of the Au25 clusters that correspond to so-called superatom D and P orbitals, respectively.

Magnetic circular dichroism in real-time time-dependent density functional theory

We apply the adiabatic time-dependent density functional theory to magnetic circular dichroism (MCD) spectra using the real-space, real-time computational method. The standard formulas for the MCD response and its A and B terms are derived from the observables in the time-dependent wave function. We find real-time method is well suited for calculating the overall spectrum, particularly at higher excitation energies where individual excited states are numerous and overlapping. The MCD sum rules are derived and intepreted in the real-time formalism; we find that they are very useful for normalization purposes and assessing the accuracy of the theory. The method is applied to MCD spectrum of C(60) using the adiabatic energy functional from the local density approximation. The theory correctly predicts the signs of the A and B terms for the lowest allowed excitations. However, the magnitudes of the terms only show qualitative agreement with experiment.